Influence of the unsaturated zone of soil layer on the solute migration

Abstract

To design waste containment barriers, low permeability soils or manufactured liner products are required. For hydraulic barriers in waste containment system, it is recommended to use soil layer of 5 m thickness when the coefficient of the permeability is higher than 10 − 6 m/s or a layer of one meter thickness, when the coefficient of permeability is lower than 10 − 9 m/s. So some authors tried to prove analytically, that the barrier thickness can be reduced when the permeability of the soil is less than 10 − 9 m/s (e.g. 10 − 10 or 10 − 11 m/s). This suggestion does not take into account the ion sorption and the solute migration delay which can occur in an unsaturated zone and reduce the solute transfer. Infiltration tests on clayey sand and clay of Gault columns are carried out in our laboratory. TDR probes at different depth allow soil moisture and soil impedance measurements. The solute concentration in the pore water is monitored by determining the electrical conductivity from the soil impedance. A lead slag leachate with an electrical conductivity of 5.3 S/m and a pH 6.3 is used for the infiltration tests. The clayey sand column is percolated during the infiltration test. But the moisture takes one year to reach the clay of Gault column bottom. The electrical conductivity and the moisture become uniform in clayey sand column after 15.5 h. The maximum value of the electrical conductivity is not yet reached in the clay column after 500 days. The forms of the electrical conductivity profiles prove that the leachate solute migration is dispersive–convective along the clayed sand column and only dispersive along the clay column. The soil impedances measured with TDR probes are used to assess the solute retained in the soil layer. These measurement results show that 300 days are needed to reach the solute retention capacity of the 25 mm thickness of the clay. The thickness of 475 mm of the clayey sand reaches its solute retention capacity after 15.5 h. These results prove that the efficiency of liner materials does not depend only on hydraulic behaviour. The use of low permeability materials reduces the water flow but a sufficient thickness is required to reduce the solute migration. The delay in the solute transfer is increased by the thickness of the unsaturated zone.